1. Field of the Invention
This invention is concerned with a fuel stack having a non-circulating, replenishable electrolyte. More particularly, the invention is concerned with a fuel cell which is one of the components of a stack of fuel cells.
The invention is particularly concerned with electrochemical cells utilizing an electrolyte which is intended to fill up a porous matrix to obtain a more uniform pressure with less weight of acid or electrolyte pressing down onto the matrix.
Acid or other electrolyte is supplied to each matrix in a stack of fuel cells at a uniform, low pressure so that the matrix can either be filled initially or replenished with acid, lost in the operation of the cell, without exceeding the bubble pressure of the matrix or the flooding pressure of the electrodes on either side. Acid control to each cell is achieved by restricting the opening in the acid fill holes with the matrix which is to be wet.
The present invention is more particularly concerned with the acid fill of a phosphoric acid fuel cell stack in which no external controls are necessary to control the flow of acid to the matrix or other fuel cell components. Further, it is not necessary with the present invention to take the fuel stack out of operation in order to maintain the matrix suitably wetted with the electrolyte or to reverse or change the position of the fuel stack.
2. Description of the Prior Art
Acid in a typical phosphoric acid fuel cell is contained in a thin porous member, referred to as the matrix. The matrix is located between two gas electrodes of the cell. Once the cell is assembled, acid additions are made to the matrix from an acid reservoir channel. At either end of this channel are located fill holes which in an assembled stack communicate with each other and form a continuous pipe through the stack.
The acid in each matrix of each cell must be maintained in a saturated condition (or a continuous film which cannot be perforated by the gases flowing on either side of the electrodes abutting the matrix) during its operation. Any excess acid should reside in the acid reservoir, but it should not be allowed to flood the electrodes and subsequently the process gas channels on the reverse side of the electrodes.
In a stack of more than a few cells, it has been the practice to add acid to the cells only when the cells are in a vertical position, although the cells operate in a horizontal position; the acid pressure to each cell can thus be maintained at a low and equal value to each matrix. If additions are made to cells held in a horizontal position, and the acid fill holes in the stack would be filled completely, therefore, each cell matrix would have a different acid pressure depending on its location in the stack. For a stack in which it is desired to limit the bubble pressure of the matrix to, for example, six inches of water, only a small number of cells could be incorporated without causing undesirable flooding of components of the lower cells. When the acid is added to cells arranged in a vertical position, then the addition to a dry matrix or replacement to a wet matrix is further restricted by the height of the matrix, above the acid reservoir and the physical properties of the matrix and the phosphoric acid electrolyte.
Reference is made to U.S. Pat. No. 3,748,179 to Bushnell which is incorporated herein by reference and is directed to a "Matrix-type" of fuel cell with circulated electrolyte. In general, the cell includes an anode, a cathode, a matrix with an ion-conductive electrolyte between the anode and cathode, and porous pins are provided to conduct the electrolyte to the matrix. Oxidant, fuel and electrolyte are applied to the cell at one end thereof by pumping and withdrawn from another end opposite the one end. Each cell forming the stack of cells has a separate inlet for the electrolyte at the said one end, and a separate outlet for the electrolyte at the other end. Flow of the electrolyte is direct from one end to the other with the only circulation taking place through the porous pins. The electrolyte flows into the electrolyte chamber of each individual cell when they are stacked in a vertical condition.
U.S. Pat. No. 4,168,349 to Buzzelli, which is incorporated by reference, is directed to an Iron/Air Battery System having Circulating Electrolyte and a Horizontal Cell Configuration. Each cell of the horizontally disposed stack of cells includes its own individual electrolyte inlet and electrolyte outlet with the electrolyte being pumped in a horizontal direction through each cell. The electrolyte supplied to one cell does not circulate through the next adjacent cell, and the outlet is larger than the inlet so that the electrolyte is pulled out of the cell, and the outlet cross-sectional area is between 1.25 to 5 times the area of the inlet cross-sectional area.
U.S. Pat. No. 3,905,832 to Trocciola, which is incorporated by reference, discloses a fuel cell structure containing an electrolyte reservoir within the cell structure for supplying electrolyte as needed to the conventional matrix.
U.S. Pat. No. 3,615,845 to Gray discloses a fuel cell in which the electrolyte is wicked through capillary conduits for even distribution along the length of the matrices between the anodes and cathodes. Make up electrolyte is to be fed to the matrices during long term cell operation at whatever rate the matrices will absorb the electrolyte.
U.S. Pat. No. 3,814,631 to Warszawski et al. discloses a framed electrode with electrolyte supply orifices on one side and drainage orifices on the other side. The channels from each of the orifices have a cross-sectional area which varies relative to the other channels in proportion to their respective lengths to equalize the pressure drop through the channels.
U.S. Pat. No. 3,926,676 to Frie et al. discloses a battery formed from a plurality of cells in which the main electrolyte inlet channel and outlet channels have a constant uniform cross-section with the outlet channel being the larger of the two with the individual cells having its electrolyte chamber connected with its own supply passage, which passages may have different cross-sections to vary the supply pressure of the electrolyte flow to the individual cells.
It is therefore an object of the invention to provide a fuel cell which forms part of a fuel stack that provides for the continuous supply of electrolyte to the matrix.
A further object of the invention is the provision of a method and system to restrict the flow rate in a stack of fuel cells so that the flow rate of the electrolyte is maintained substantially evenly to each fuel cell.
Yet another object of the invention is the provision of a stack of fuel cells in which an electrolyte reservoir is maintained at selected locations with respect to the fuel cell stack.
A further object of the invention is to provide a flow mechanism which relies upon a small pressure head in each cell to saturate the area around the matrix and then in turn to saturate all of the matrix.